This invention relates to intercom systems and more particularly to an intercom system for a key telephone system.
The telephone industry presently offers a K1A2 key telephone system consisting of a basic pre-wired mounting package that is designated as a key service unit. It contains provisions to receive associated "plug-in" units in varying combinations to meet subscriber requirements.
One feature of this key system is a known rotary dial intercom unit. The intercom unit provides a single common talking path for all stations in the system. When a call is initiated, the system provides a steady busy lamp signal at all locations and causes a single burst or ringing signal at the called station only. If the called station does not answer, the calling station must re-dial the code of the called station in order to initiate a second ring burst at the called station. This requirement of re-dialing, of course, is a disadvantage.
A first alternative to overcome the disadvantage of the known intercom unit is set forth in my co-pending application (Case 1), Ser. No. 683,877, filed May 6, 1976, in which a control circuit is provided including a mechanical interrupter which provides a plurality of interrupted ringing signal bursts when one of a plurality of intercom stations goes off-hook and first initiates a call to another of a plurality of intercom stations. The interrupter also provides an interrupted voltage to cause a button lamp at the another of the plurality of stations to flash. A call selector circuit and call routing circuit are coupled to the control circuit and the plurality of stations to provide a connection between the one and the another of the plurality of stations to enable the interrupted ringing signal bursts to be conducted on the connection to the another of the plurality of stations. Logic circuitry is coupled to the plurality of intercom stations and the control circuit to detect when the one and the another of the plurality of stations are both off-hook. When this condition is detected, the interrupted ringing signal bursts are stopped and the button lamp is caused to glow steadily.